MRI signals are sensitive to physiological alterations such as changes in cerebral blood flow (CBF), volume (CBV) and oxygenation. As a consequence, MRI methods can be designed that produce blood-oxygenation-level-dependent (BOLD) image contrast. The possibility of detecting such effects has stimulated a boon in the field of functional MRI (fMRI) of the brain, where neuronal activation is reflected in slight focal increases in signal intensity. Despite great progress in understanding some of the mechanisms of these BOLD signal changes, the technique is often criticized because no exact equations relating the measure MRI effects to basic physiological parameters, such as CBF, CBV, oxygen metabolic rate, hematocrit, and arterial oxygen saturation, have been established. The applicants have recently developed a general theory that can quantitatively explain spin-echo (SE) relaxation effects (R2) in terms of hemoglobin deoxygenation and oxygen extraction ratios (OER). This theory has to be tested rigorously using experiments in which hemoglobin deoxygenation (Aim 1) and OER (Aims 2,3) are well understood and can be controlled, after which it can be applied to determine OER effects in fMRI (Aim 4). The aim is to quantitatively measure R2 and the SE signal intensities of water as a function of the inter-echo time spacing in the NMR pulse sequence and as a function of field strength for the following conditions: At different hemoglobin oxygenation levels and hematocrits in isolated blood (AIM); As a function of oxygenation in vivo in the cat brain (AIM 2), and in vivo in the human brain (AIM 3). These experiments will also include simultaneous determination of arterial oxygen saturation, pH, and blood gases (AIM 1-3), as well as measurement of arteriovenous differences and absolute blood flow (microspheres) in the animals (AIM 2). Finally, these relaxation rates and arterial oxygenation and blood gases will be measured during visual stimulation in humans at field strengths of 1.5T and 4.0T (AIM 4). These efforts should lead to a better understanding of the physiological mechanisms underlying the fMRI signal changes in neuronal activation and allow quantitation of cerebro-haemodynamic parameters to be utilized for high resolution mapping of structure/function relationships in intact brain. This understanding should facilitate optimal design of fMRI experiments in terms of the most suitable MRI pulse sequence parameters to obtain maximum effects.